Like everyone here I remember parts of the story. I believe Buffy and Spike are a couple an they are estranged from the scooby gang. They are in Los Angeles where they run into Wes (or maybe they seek him out.) He explains about Conner, the portal, and the falling out with AI, so they decided to mount a rescue.

Not sure if this is enough to go by... But if not... I'll take any story where they save or end up raising Conner. Cheers!

I know this one, it's one of two with wesley and buffy it's on AO3, if you search you will find it, sorry that all I remember.